Microwave bandstop filter structures, such as those employed for multi-port antenna feeds, have conventionally been constructed using custom or precision designs, or by using approximate waveguide-configured structures. While conventional exact design filter structures can meet both performance and reduced volume packaging objectives, their extremely narrow dimensional tolerances require that the filters be electro-formed, which considerably increases the cost of manufacture. Larger waveguide-configured structures, on the other hand, are less expensive to manufacture but, because of their size, usually do not meet packaging requirements of the associated antenna system.
More particularly, as diagrammatically illustrated in FIG. 1, a compact precision design bandstop filter is typically formed by electroplating a metal, usually copper, although nickel is sometimes employed, onto a mandrel. The mandrel is preconfigured to provide a plurality of successive, diametrically opposed pairs of generally rectangularly shaped lumped bandstop filter elements or E-plane shorted stubs 11, which effectively function as a distributed series of varying impedances along a longitudinal waveguide-configured section 13. Each diametrically opposed pair of E-plane rectangular waveguide segments, or shorted stubs, is oriented transverse to the longitudinal waveguide section and has an effective electrical length of one-quarter wavelength of a prescribed frequency to be excised from a band of signals over which the filter structure is intended to operate.
As one traverses the length of the longitudinal waveguide section, the pairs of stubs, which are spaced an odd number of quarter wavelengths apart, vary in impedance level, as do the longitudinal waveguide sections. The impedances of the stubs generally decrease from the center of the filter structure to the outer edges and their cross-sections or widths W become narrower. In terms of a practical design for a narrow band Ku band filter, it is not uncommon for the widths of the narrower E-plane stubs to be on the order of fifteen to twenty mils. With a stub aspect ratio on the order of ten-to-one (associated with the required quarter-wavelength depth of the E-plane short), producing a filter structure having E-plane stubs of such dimensions have been achieved only by electro-forming the filter on a preshaped mandrel.
The high cost of precision electro-forming constitutes a significant impediment to the proliferation of a wide variety of small aperture earth terminals in today's satellite communication environment, where minimizing component cost and maintenance expenses are principal motivators to the system designer. Not only do electro-formed components cost more to manufacture, but because the metal employed (e.g. the above-referenced copper or nickel) to electro-form such parts is not the same as that of most of the remaining hardware components of the system, particularly waveguide sections made of aluminum, there is often a metallic mismatch at the joints between an electro-formed part and the electromagnetic energy `plumbing` to which the part is connected, which subjects the hardware to potential `mechanical insertion loss` over a period of use.
In a larger, waveguide-configured structure, such as that diagrammatically shown in FIG. 2 (which corresponds generally to FIG. 12.01-1(b) of the text by G. L. Matthaei et al, entitled "Microwave Filters, Impedance-Matching Networks, and Coupling Structures," published by McGraw-Hill Book Co., 1964), a plurality of generally rectangularly shaped lumped bandstop filter elements 21 are individually distributed along a longitudinal section of rectangular waveguide 23. Each bandstop filter element 21 comprises a respective E-plane waveguide segment or stub, oriented transverse to the axis of the longitudinal waveguide section and having an effective electrical length L of one-half wavelength of a prescribed frequency to be excised from a prescribed band of signals with which the filter structure is intended to operate.
In order to prevent adjacent filter elements from interacting with one another, the bandstop filter elements 21 are spaced apart from one another along the waveguide section 23 at successive intervals corresponding to three-quarters of the wavelength of the center frequency of the filter's operational bandwidth, which implies a relatively large lengthwise dimension of the filter. In such a waveguide-configured structure, each half-wavelength waveguide E-plane segment is electromagnetically coupled to the longitudinal waveguide section by way of an aperture or iris 25 formed in a broadwall of the longitudinal waveguide section 23. The sizes of the irises are tailored to adjust the effective impedances of the stubs to approximate the performance of the exact design of FIG. 1. The ends 27 of the lumped elements 21 comprise conductive walls which effectively provide a shorted termination for each filter element.
Now, although the waveguide-configured bandstop filter architecture of FIG. 2 is less expensive to manufacture than the electro-formed configuration of FIG. 1, its substantial size (overall physical length) makes this structure unsuitable for current compact packaging requirements.